1. Field of the Invention
This invention relates generally to video interfaces and, more particularly, to a programmable video interface that operates based on digital programming data.
2. Discussion of the Related Art
In many military and commercial aviation environments, it is necessary to accurately convert analog video signals into digital video signals for a variety of purposes. One such application is the identification system used by air traffic control or military aircraft. In these systems, a processor will analyze parameters such as the pulse width, pulse amplitude and position of the received digitized signal to determine the identity of the transmitting party. In order to generate the necessary digital signal for use by the processor, an analog input signal must be scaled and digitized based on a predetermined transfer characteristic. It is desirable that this transfer characteristic map the output of the log amp to the full scale input range of the A/D converter to ensure that the maximum available resolution is attained. It is not uncommon for the tolerance of this mapping to be specified at less than xc2x14%.
This requirement for accurate conversion of the analog input signal requires precise setting of amplifier gains and offsets. Conventional systems scale the output of the log amplifier, with a differential amplifier, to drive an analog to digital (A/D) converter which provides the necessary conversion based on the gain and offset settings. The gain of the differential amplifier is set by the ratio of resistance in its input and feedback networks. Similarly, the offset of the differential amplifier is set by the ratio of resistance in a voltage divider.
The prohibition of the use of potentiometers in military avionics circuitry creates a substantial problem in the design, manufacture and testing of the scaling module. This is because the accuracy requirements associated with video interfaces can exceed the production tolerances of the individual components, requiring xe2x80x9cselect-in-testxe2x80x9d parts during manufacture. In particular, typical production tolerances for the log amplifier are on the order of 10%, which in itself exceeds the total error budget for the project. The costs associated with having to inventory a range of precision components (to insure that the particular value is available for the select-in-test process), the test and retest times, and labor can also be a significant part of the cost of production. It is therefore desirable to provide a programmable video interface that can meet the predetermined transfer characteristics and associated tolerances which are common to video applications allowing the automation of adjustments in lieu of the manual select-in-test process.
In accordance with the teachings of the present invention, a programmable video interface is disclosed. In one embodiment, the video interface includes a scaling module and a programming module. The scaling module converts an analog input signal into a digital output signal based on a plurality of analog programming signals. The programming module generates the plurality of analog programming signals, where the analog programming signals maintain operation of the scaling module at a predetermined transfer characteristic and an associated tolerance. The programming module generates the plurality of analog programming signals based on digital programming data. Providing a programming module that operates based on digital programming data enables the elimination of conventional voltage divider networks and accompanying inaccuracies.
The programming module utilizes a first digital to analog (D/A) converter to generate an analog offset signal based on digital offset data. A second D/A converter generates an analog gain signal, which sets the full scale voltage range of the A/D converter based on digital gain data. The plurality of analog programming signals include the analog offset signal and the analog gain signal, where the analog offset signal and the analog gain signal maintain operation of a programmable video interface scaling module at a predetermined transfer characteristic and an associated tolerance.
Further in accordance with the present invention, a method for programming a video interface programming module is disclosed. The method includes the step of determining digital offset data for the programming module, where the digital offset data corresponds to a predetermined transfer characteristic for the video interface. The programming module transmits an analog offset signal to a scaling module based on the digital offset data. The method further provides for determining digital gain data for the programming module, where the digital gain data corresponds to the predetermined transfer characteristic. The programming module transmits an analog gain signal to the scaling module based on the digital gain data. The scaling module converts an analog input signal into a digital output signal based on the predetermined transfer characteristic. The digital offset data and the digital gain data are also stored to a programmable memory device.